Subject: [mobile-ip] closing issue 80: "Please put back the R bit"

From: gabriel montenegro <[email-masked]>
Date: Mon, 30 Sep 2002 01:19:38 +0200

This is an attempt to close issue 80 ("Please put back the R bit")
documented in:

  http://www.piuha.net/~jarkko/publications/mipv6/issues/issue80.txt

BACKGROUND
==========
In draft-14 there was an 'R' bit in the Binding Update.
The purpose of this bit was to enable support for mobile
routers (as opposed to mobile nodes). These are systems
that (1) use MIPv6 for their own mobility, and (2) provide
routing services for other nodes while away from home.

The 'R' bit was intended only for MN-HA binding updates
(not for MN-CN) and was defined as follows:

      Router (R)

         The Router (R) bit, when set, indicates that the sending
         mobile node is a router.  This bit is only valid when the
         Home Registration (H) bit is also set, and MUST NOT be set
         otherwise.  This bit is saved in the home agent's "home
         registration" Binding Cache entry for the mobile node, and
         is copied into the corresponding bit in all proxy Neighbor
         Advertisement messages sent on behalf of this mobile node by
         the home agent using this Binding Cache entry.

This bit is used by the home agent to know whether to
issue proxy Neighbor Advertisements with the R bit on
or off, and disappeared in rev 15 of the draft. Notice that at the
time of those drafts the binding update was still a destination option.

The R bit was taken out following a decision to not include any
support for mobile routers in the base MIPv6 specification.
This activity, albeit important and worthwhile, is better
carried out elsewhere. The base MIPv6 specification focuses
on node mobility.

It is important to clarify that this distinction applies while
the node is away from home. In particular, while at home,
the node may also be a router. Therefore, it is important to
distinguish between these two cases:

1. router-when-at-home
2. router-when-away-from-home

The base MIPv6 specification (draft-ietf-mobileip-ipv6*.txt)
is not concerned with #2. At the same time, MIPv6 must not
preclude #1, that is, while at home, a node may also be
a router.

ISSUE
=====
First of all, the issue is *not* that some folks want to bring
back any mobile router functionality (case #2 above) into
MIPv6. This is clearly understood to be out of scope for
MIPv6.

Some folks want this bit back because they claim it is required
to support nodes that perform as a router while at home
(#1 above). This is obviously not a problem because
at home the mobile node issues its own neighbor
advertisements per rfc2461, and turns on the R bit or not
by itself. Furthermore, there is no issue when the node
moves away from home.

In this case, the home agent issues the neighbor
advertisements on the mobile node's behalf. Given that
MIPv6 does not support mobile routers, any neighbor
advertisements sent out by the home agent
on behalf of the mobile node away from home
MUST NOT have the R bit set.

This is accomplished by not assigning such a bit
in the binding update (as done starting in rev
15 of the draft). Hence, the home agent is obligated
to issue proxy neighbor advertisements with a zeroed
out 'R' bit. Nodes on the home link detect the transition
from 1 to 0 and modify their neighbor caches accordingly.

Another issue is that some folks claim this bit must
be at least reserved now to guarantee that the
protocol will be extensible for mobile routers in the future. Given
that rev 18 of the draft includes 11 currently unassigned bits, the
probability of running out of bits to assign is negligible, particularly,
because work to define extensions for mobile routers is already under way
(including the definition of the 'R' bit).
See draft-kniveton-mobrtr-02.txt and the NEMO proposed working group.

PROPOSAL
========
Reject this issue. No changes to the text are required.

Please post any issues to the list.